There is a growing body of evidence that epigenetic changes may not simply be a symptom of aging but an underlying cause (Brunet & Berger, 2014). In budding yeast, the reorganization of chromatin and resulting alterations in gene expression are known contributors to aging. In mammals, similar processes appear to be associated with aging but whether they truly cause aging is not yet known. In the last round of this grant, we provided evidence that chromatin factors such as Sir2 in yeast and SIRT1 in mammals, are key players in a process that suppresses epigenetic change and aging in eukaryotes. To provide a direct test of whether epigenetic change can actually cause aging in a mammal, we have generated a new mouse model called ?ICE?, for inducible changes in the epigenome. This mouse carries a tamoxifen- inducible endonuclease that induces non-mutagenic, site-specific DNA breaks (DSBs) at a few sites in the mammalian genome, thereby stimulating epigenetic change. We can induce change in any tissue at any time in a mouse's lifespan, then switch the system off, and monitor the effects on aging. Results to date are consistent with alterations to the epigenome being a fundamental cause of aging in mammals. This study is important, because it may explain long-standing questions about aging, such as why DNA repair defects and nuclear Lamin mutations both result in diseases that resemble premature aging. It will also address whether aging is uni-directional or reversible, and provide tools to screen for potent longevity interventions and to ?humanize? mouse models of disease. In Aim 1, we utilize fibroblasts from ICE mice to elucidate the mechanisms that drive epigenetic changes and the effects they have on the resilience of cells to DNA damage and senescence. Advanced genomic technologies will allow us to map protein-binding, histone modifications, transcription profiles, DNA methylation patterns, and the evolution of nuclear architectures in 3D during aging. In Aim 2, we test cause and effect in a living mammal by inducing epigenetic change in young mice and observing the effects on nuclear organization and the aging process. In Aim 3, we test if epigenetic decay can be prevented or reversed using the latest genetic or pharmacological approaches. Together, this work will provide new information about whether alterations in chromatin structure promote aging in eukaryotes and explore ways that this knowledge can be exploited to improve the human condition.